Aerospace vehicles are used for a variety of different purposes. For example, aircraft (e.g., both manned and unmanned), spacecraft (e.g., both manned and unmanned), unmanned aerial vehicles (“UAV”) and satellites are in widespread use for various purposes including communications, scientific research and other civil and military applications. However, many aerial missions may not be accurately fulfilled without consistently monitoring and controlling a 3-axis attitude of the vehicle.
Attitude may be described as the vehicle orientation with respect to some external reference frame. In many applications, the vehicle must be oriented to transmit signals in particular directions and/or to receive signals from specifically located sources. Without accurate control over the vehicle 3-axis attitude, the transmission and/or reception of such signals may be hindered and, at times, may be impossible.
Such control may require systems for 3-axis attitude, which generally include one or more star trackers. During normal operation, star trackers may provide attitude-related information. As there may be inherent, and time-varying, errors from star trackers, it may be necessary to constantly estimate such errors, in order to compensate for them. One method of doing so is to identify stars tracked by the star tracker and correlate the star tracker position measurements of the stars with known positions of the same stars, as listed in an onboard star catalog.
When a star tracker is pointed at a region in the sky where the attitude determination system cannot identify a minimum number of stars for a predetermined period of time, the vehicle attitude may be properly maintained and/or a vehicle fault may occur. Such a region may be defined to be a “dry spot”.
When the star tracker cannot identify a minimum number of stars for a predetermined period of time, a dry spot may occur and the vehicle attitude may not be properly maintained and/or a vehicle fault may occur.
Accordingly, those skilled in the art continue with research and development efforts in the field of star trackers to minimize or eliminate the occurrences of less than a minimum number of stars being identified by a star tracker.